Arsenic Contamination Threatens Water in Hanoi

Arsenic-laden sediment that washed down from the Himalayas eons ago underlies vast stretches of Asia, from Pakistan to China. When it gets into underground aquifers, as has happened in Bangladesh, it can contaminate public water supplies and cause illness and death.

Now researchers say arsenic is leaching into a major drinking-water aquifer that serves Hanoi, Vietnam. The culprit, they say, is pumping from private wells, which is draining that aquifer and drawing water from others that contain arsenic.

But the poison is moving more slowly than scientists had feared, and the city still has years or even decades to take protective measures.

The study, by Vietnamese scientists in collaboration with researchers from Columbia University’s Lamont-Doherty Earth Observatory and elsewhere, was “the first to show that a previously clean aquifer has been contaminated,” said the lead author, Alexander van Geen, a geochemist at Columbia. It was published by the journal Nature.

Whether arsenic leaches into underground water depends on the balance of iron and decaying plant material in the aquifer. The chemical process is only partly understood, but iron seems to bind to the arsenic, while carbon in the decaying plants slowly dissolves the iron, and the released arsenic flows into the plume.

Those elements flow through the sediments much more slowly than water does. The team’s 31 research wells in areas of Hanoi near the Red River showed that aggressive pumping of a safe aquifer over the last 20 years had pulled water more than a mile in from a contaminated one. But over the same period, the plume of arsenic contamination — indicated by a streak of gray sand through the safer rust-colored sediments — had moved less than 400 feet.

Arsenic in some areas is 10 to 50 times higher than levels considered safe, Vietnamese officials said. The city plans to install a filter plant, but many Hanoi residents rely on the private wells that are making matters worse.

A version of this article appears in print on , Section D, Page 5 of the New York edition with the headline: Arsenic Contamination Threatens Water in Hanoi. Order Reprints | Today’s Paper | Subscribe